Observations of gamma‐ray bursts by the Fermi satellite, capable of detecting photons in a very broad energy band: 8 keV to >300 GeV, have opened a new window for the study of these enigmatic explosions. It is widely assumed that photons of energy larger than 100 MeV are produced by the same source that generated lower energy photons – at least whenever the shape of the spectrum is a Band function. We report here a surprising result – the Fermi data for a bright burst, GRB 080916C, unambiguously shows that the high‐energy photons (≳102 MeV) were generated in the external shock via the synchrotron process, and the lower energy photons had a distinctly different source. The magnetic field in the region where high‐energy photons were produced (and also the late‐time afterglow emission region) is found to be consistent with shock compressed magnetic field of the circum‐stellar medium. This result sheds light on the important question of the origin of magnetic fields required for gamma‐ray burst afterglows. The external shock model for high‐energy radiation makes a firm prediction that can be tested with existing and future observations.

We present a systematic study on magnetic fields in Gamma-Ray Burst (GRB) external forward shocks (FSs). There are 60 (35) GRBs in our X-ray (optical) sample, mostly from Swift. We use two methods to study ǫ B (fraction of energy in magnetic field in the FS). 1. For the X-ray sample, we use the constraint that the observed flux at the end of the steep decline is ≥ X-ray FS flux. 2. For the optical sample, we use the condition that the observed flux arises from the FS (optical sample light curves decline as ∼ t −1 , as expected for the FS). Making a reasonable assumption on E (jet isotropic equivalent kinetic energy), we converted these conditions into an upper limit (measurement) on ǫ B n 2/(p+1) for our X-ray (optical) sample, where n is the circumburst density and p is the electron index. Taking n = 1 cm −3 , the distribution of ǫ B measurements (upper limits) for our optical (X-ray) sample has a range of ∼ 10 −8 − 10 −3 (∼ 10 −6 − 10 −3 ) and median of ∼ few × 10 −5 (∼ few × 10 −5 ). To characterize how much amplification is needed, beyond shock compression of a seed magnetic field ∼ 10µG, we expressed our results in terms of an amplification factor, AF , which is very weakly dependent on n (AF ∝ n 0.21 ). The range of AF measurements (upper limits) for our optical (X-ray) sample is ∼ 1 − 1000 (∼ 10 − 300) with a median of ∼ 50 (∼ 50). These results suggest that some amplification, in addition to shock compression, is needed to explain the afterglow observations.

We consider a sample of ten GRBs with long lasting ( 10 2 sec) emission detected by Fermi/LAT and for which X-ray data around 1 day are also available. We assume that both the X-rays and the GeV emission are produced by electrons accelerated at the external forward shock, and show that the X-ray and the GeV fluxes lead to very different estimates of the initial kinetic energy of the blast wave. The energy estimated from GeV is on average ∼ 50 times larger than the one estimated from X-rays. We model the data (accounting also for optical detections around 1 day, if available) to unveil the reason for this discrepancy and find that good modelling within the forward shock model is always possible and leads to two possibilities: either the X-ray emitting electrons (unlike the GeV emitting electrons) are in the slow cooling regime or ii) the X-ray synchrotron flux is strongly suppressed by Compton cooling, whereas, due to the Klein-Nishina suppression, this effect is much smaller at GeV energies. In both cases the X-ray flux is no longer a robust proxy for the blast wave kinetic energy. On average, both cases require weak magnetic fields (10 −6 B 10 −3 ) and relatively large isotropic kinetic blast wave energies 10 53 erg < E 0,kin < 10 55 erg corresponding to large lower limits on the collimated energies, in the range 10 52 erg < E θ,kin < 5 × 10 52 erg for an ISM environment with n ∼ 1cm −3 and 10 52 erg < E θ,kin < 10 53 erg for a wind environment with A * ∼ 1. These energies are larger than those estimated from the Xray flux alone, and imply smaller inferred values of the prompt efficiency mechanism, reducing the efficiency requirements on the still uncertain mechanism responsible for prompt emission.

We analyze the >100-MeV data for three gamma-ray bursts (GRBs) detected by the Fermi satellite (GRBs 080916C, 090510, 090902B) and find that these photons were generated via synchrotron emission in the external forward shock. We arrive at this conclusion by four different methods as follows. (1) We check the light curve and spectral behaviour of the >100 MeV data, and late-time X-ray and optical data, and find them consistent with the socalled closure relations for the external forward shock radiation. (2) We calculate the expected external forward shock synchrotron flux at 100 MeV, which is essentially a function of the total energy in the burst alone, and it matches the observed flux value. (3) We determine the external forward shock model parameters using the >100 MeV data (a very large phase space of parameters is allowed by the high-energy data alone), and for each point in the allowed parameter space we calculate the expected X-ray and optical fluxes at late times (hours to days after the burst) and find these to be in good agreement with the observed data for the entire parameter space allowed by the >100 MeV data. (4) We calculate the external forward shock model parameters using only the late-time X-ray, optical and radio data and from these estimate the expected flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and find that to be entirely consistent with the high-energy data obtained by Fermi/LAT. The ability of a simple external forward shock, with two empirical parameters (total burst energy and energy in electrons) and two free parameters (circumstellar density and energy in magnetic fields), to fit the entire data from the end of the burst (1-50 s) to about a week, covering more than eight decades in photon frequency ->10 2 MeV, X-ray, optical and radio -provides compelling confirmation of the external forward shock synchrotron origin of the >100 MeV radiation from these Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that the magnetic field required in these GRBs is consistent with shockcompressed magnetic field in the circumstellar medium with pre-shocked values of a few tens of µG.

Binary neutron star mergers are considered to be the most favourable sources that produce electromagnetic (EM) signals associated with gravitational waves (GWs). These mergers are the likely progenitors of short duration gamma-ray bursts (GRBs). The brief gamma-ray emission (the ‘prompt’ GRB emission) is produced by ultrarelativistic jets, as a result, this emission is strongly beamed over a small solid angle along the jet. It is estimated to be a decade or more before a short GRB jet within the Laser Interferometer Gravitational-Wave observatory (LIGO) volume points along our line of sight. For this reason, the study of the prompt signal as an EM counterpart to GW events has been sparse. We argue that for a realistic jet model, one whose luminosity and Lorentz factor vary smoothly with angle, the prompt signal can be detected for a significantly broader range of viewing angles. This can lead to an ‘off-axis’ short GRB as an EM counterpart. Our estimates and simulations show that it is feasible to detect these signals with the aid of the temporal coincidence from a LIGO trigger, even if the observer is substantially misaligned with respect to the jet.